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<a name="s390-Operands"></a>
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<a name="Instruction-Operands"></a>
<h4 class="subsubsection">9.38.3.3 Instruction Operands</h4>
<a name="index-instruction-operands_002c-s390"></a>
<a name="index-s390-instruction-operands"></a>

<p>Instruction operands can be grouped into three classes, operands located
in registers, immediate operands, and operands in storage.
</p>
<p>A register operand can be located in general, floating-point, access,
or control register. The register is identified by a four-bit field.
The field containing the register operand is called the R field.
</p>
<p>Immediate operands are contained within the instruction and can have
8, 16 or 32 bits. The field containing the immediate operand is called
the I field. Dependent on the instruction the I field is either signed
or unsigned.
</p>
<p>A storage operand consists of an address and a length. The address of a
storage operands can be specified in any of these ways:
</p>
<ul>
<li> The content of a single general R
</li><li> The sum of the content of a general register called the base
register B plus the content of a displacement field D
</li><li> The sum of the contents of two general registers called the
index register X and the base register B plus the content of a
displacement field
</li><li> The sum of the current instruction address and a 32-bit signed
immediate field multiplied by two.
</li></ul>

<p>The length of a storage operand can be:
</p>
<ul>
<li> Implied by the instruction
</li><li> Specified by a bitmask
</li><li> Specified by a four-bit or eight-bit length field L
</li><li> Specified by the content of a general register
</li></ul>

<p>The notation for storage operand addresses formed from multiple fields is
as follows:
</p>
<dl compact="compact">
<dt><code>Dn(Bn)</code></dt>
<dd><p>the address for operand number n is formed from the content of general
register Bn called the base register and the displacement field Dn.
</p></dd>
<dt><code>Dn(Xn,Bn)</code></dt>
<dd><p>the address for operand number n is formed from the content of general
register Xn called the index register, general register Bn called the
base register and the displacement field Dn.
</p></dd>
<dt><code>Dn(Ln,Bn)</code></dt>
<dd><p>the address for operand number n is formed from the content of general
regiser Bn called the base register and the displacement field Dn.
The length of the operand n is specified by the field Ln.
</p></dd>
</dl>

<p>The base registers Bn and the index registers Xn of a storage operand can
be skipped. If Bn and Xn are skipped, a zero will be stored to the operand
field. The notation changes as follows:
</p>
<div class="display">
<table>
<thead><tr><th width="30%"><pre class="display">full notation</pre></th><th width="30%"><pre class="display">short notation</pre></th></tr></thead>
<tr><td width="30%"><pre class="display">Dn(0,Bn)</pre></td><td width="30%"><pre class="display">Dn(Bn)</pre></td></tr>
<tr><td width="30%"><pre class="display">Dn(0,0)</pre></td><td width="30%"><pre class="display">Dn</pre></td></tr>
<tr><td width="30%"><pre class="display">Dn(0)</pre></td><td width="30%"><pre class="display">Dn</pre></td></tr>
<tr><td width="30%"><pre class="display">Dn(Ln,0)</pre></td><td width="30%"><pre class="display">Dn(Ln)</pre></td></tr>
</table>
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